1. Field of the Invention
This invention relates to a nano-crystal diamond film formed of fine diamond crystal of the order of nanometers. This invention also relates to a method of manufacturing such a nano-crystal diamond film and to various kinds of devices using such a nano-crystal diamond film.
2. Description of the Related Art
A type of diamond in which carbon atoms are bonded to each other through covalent bond by way of sp3 hybrid orbital is widely known to have, due to its high bonding energy, specific physical properties that cannot be derived from other kinds of materials. In recent years, it has been made possible to synthesize a film-like diamond (diamond film) of high quality at low temperatures by making use of a chemical vapor-phase deposition method (CVD method). In this film-forming method, a thermal filament CVD method or a microwave CVD method is generally employed.
According to these methods of forming a diamond film, it is possible to form a monocrystalline diamond film as a homoepitaxial film on a diamond substrate (natural or high-pressure synthetic diamond). On the other hand, it is also possible to form a polycrystalline diamond film on a silicon, metal or quartz substrate.
Although it is certainly possible to synthesize a monocrystalline diamond film of high quality on a diamond substrate, it is required to employ, as a substrate, natural diamond or high-temperature/high-pressure synthetic diamond. In this case, the size of the substrate is limited at most to about 10 mm×10 mm at present.
On the other hand, a polycrystalline diamond film can be formed on a substrate of relatively large size using silicon. However, since the diamond film to be obtained is a polycrystal, the surface of diamond film is highly rough. Namely, since the grain diameter of crystal particles constituting the polycrystalline diamond film is as large as 1-10 μm, the surface of diamond film is caused to become prominent in roughness.
Furthermore, since it is required to heat a substrate up to a high temperature of 800° C. or more in the synthesis of both a homoepitaxial film and polycrystalline diamond film, it is considered necessary to employ a high heat-resistant substrate such as expensive diamond, single crystal silicon or quartz. Therefore, it is impossible to employ a substrate which is suited for forming a diamond film of large area at low cost such as a glass substrate or a polymer substrate to be used for a display for example.
As described above, all of the conventional diamond films are not suited for practical use because of the cost of the substrate material and of the difficulties to increase the size thereof.
Under the circumstances, it is now desired to develop a nano-crystal diamond film which is formed of fine crystal grains of the order of nanometers and hence is smooth in surface, and which can be formed at low temperatures on a substrate having a large surface area and made of one of various kinds of materials. However, no one has succeeded, as yet, in discovering such a film and a method of forming such a film.
Incidentally, there is known a technique of manufacturing diamond having a particle diameter ranging from 20 to 50 nm, wherein carbon nanotubes are treated at a high temperature of 1600° C. or more under a high pressure of 10 GPa or more to manufacture the diamond. However the product thus prepared is not in the form of film but merely in the form of particles (for example, JP Patent Laid-open Publication (Kokai) No. 2002-66302).
Objects of the present invention are to provide a nano-crystal diamond film which is formed of fine crystal grains of the order of nanometers and hence is smooth in surface, and which can be formed on substrates of various kinds of materials at a temperature specifically suited for each of the substrates, and also to provide a method for manufacturing the nano-crystal diamond film.